hi guys a shop owner here is trying to sell me his 2nd hand ( not really sure if its 2nd hand) TEIN full coilovers for $400, he said its 16 way adjustable dampening, it has a helper spring too, the coils look good but some parts have rust on them.

how do i know if its working well? and is a $400 decent price for an 2nd hand CO?

thanks